An inductosyn scale which is an electromagnetic-induction-type position detector is used as a position detection unit in various machines such as a machine tool, an automobile, and a robot. Inductosyn scales include a linear scale with a linear shape and a rotary scale with a rotary shape. The linear scale is, for example, a scale which is installed in a linear moving portion such as a linear moving shaft of a machine tool and which detects a linear movement position of the moving portion. The rotary scale is, for example, a scale which is installed in a rotating portion such as a rotary shaft of a machine tool and which detects a rotation position (rotation angle) of the rotating portion.
The linear scale and the rotary scale are both scales which detect the positions by means of electromagnetic induction of coil patterns arranged in parallel to face each other. Principles of this detection are described with reference to FIG. 4. Note that FIG. 4 includes views for explaining the principles of the linear scale, and part (a) of FIG. 4 is a view in which a slider and a scale of the linear scale are illustrated next each other, while part (b) of FIG. 4 is a view illustrating a degree of electromagnetic coupling between the slider and the scale. Although parts (a) and (b) of FIG. 4 are views illustrating the principles of the linear scale, principles of the rotary scale are the same as the principles of the linear scale, and a stator and a rotor of the rotary scale correspond to the slider and the scale of the linear scale, respectively.
As illustrated in part (a) of FIG. 4, a detection portion of the linear scale includes a slider 41 which is a primary-side plate member and a scale 42 which is a secondary-side plate member. In the linear scale, the slider 41 includes a first slider coil 43 which is a first primary-side coil and a second slider coil 44 which is a second primary-side coil. Moreover, the scale 42 includes a scale coil 45 which is a secondary-side coil. Each of these coils 43, 44, 45 is folded back and forth in a zigzag shape and is formed to have a linear shape as a whole.
The slider 41 (first slider coil 43 and second slider coil 44) and the scale 42 (scale coil 45) are arranged in parallel to face each other with a predetermined gap g maintained therebetween. Moreover, as illustrated in part (a) of FIG. 4, the position of the first slider coil 43 and the position of the first slider coil 44 are shifted from each other by a ¼ pitch (phases are shifted from each other by 90°), where one pitch of the scale coil 45 is the reference.
In the linear scale, an excitation current (alternating current) is made to flow through each of the first slider coil and the second slider coil 44. Then the degree of electromagnetic coupling between the scale coil 45 and each of the first slider coil 43 and the second slider coil 44 periodically changes as illustrated in part (b) of FIG. 4, depending on a change in a relative positional relationship between the scale coil 45 and each of the first slider coil 43 and the second slider coil 44 which is caused by the movement of the slider 41. Accordingly, an excitation voltage which periodically changes is generated in the scale coil 45.
An example of position detection is described. A first excitation current Ia as shown in the following formula (1) is made to flow through the first slider coil 43 and a second excitation current Ib as shown in the following formula (2) is made to flow through the second slider coil 44. Then an excitation voltage V as shown in the following formula (3) is generated in the scale coil 45 by electromagnetic induction between the scale coil 45 and each of the first slider coil 43 and the second slider coil 44. A peak amplitude Vp obtained by sampling the following formula (3) can be expressed as shown in the following formula (4).Ia=−I·cos(kα)·sin(ωt)  (1)Ib=I·sin(kα)·sin(ωt)  (2)V=K(g)·I·sin(k(X−α))·sin(ωt)  (3)Vp=K(g)·I·sin(k(X−α))  (4)
In these formulae, “I” is the magnitude of the excitation current, “k” is 2π/p, “p” is the length (angle in the rotary scale) of one pitch of the scale coil 45, “ω” is the angular frequency of the excitation current (alternating current), “t” is time, and “α” is the excitation position. Moreover, “K(g)” is the coupling coefficient depending on the strength of coupling in the gap g between the scale coil 45 and the pair of the first slider coil 43 and the second slider coil 44 and the like, and “X” is the displacement (movement position) of the scale.
By performing control such that the excitation position α follows the displacement X and Vp=0 is satisfied in the formula (4) described above, α=X is obtained as a detection position.